The present invention relates in general to acoustic loudspeakers and more particularly, to a unique loudspeaker which utilizes a planar diaphragm which feeds a unique waveguide structure and is magnetically biased with one or more high flux density magnetic materials with a unique physical structure. The art of the present invention is especially useful for reproducing high frequency audio signals at high power levels with a minimum of distortion.
Traditional loudspeaker designs have utilized substantially tubular voice coils or solenoids peripherally attached with a diaphragm or dome which are driven with an alternating audio feed current and immersed (at least partially) within a magnetic field. That is, based upon established electromagnetic principles, the prior art attempts to direct and place a magnetic field perpendicular to voice coil current flow in order to create a force perpendicular to both the current flow and magnetic field direction. (that is according to Ampère, the vector force is equal to the vector cross product of the current and magnetic flux density, i.e. {right arrow over (F)}={right arrow over (I)}×{right arrow over (B)}) The resultant vectorial force is desirably focused in the direction of diaphragm movement.
This prior art technique relies upon a magnetic pole of a first polarity within the solenoidal structure and a second magnetic pole of opposite polarity surrounding the solenoid with the coils of the solenoid there between. Unfortunately during operation, some of the conductive coils of the prior art voice coils are either outside of the magnetic field bias or are carrying current in a non three dimensional orthogonal direction relative to the magnetic field lines and the direction of movement of the diaphragm. That is, the prior art by its very design relies upon a fringing of the fields between north and south poles of the magnetic structure which is never quite orthogonal across the coil as a whole. This effect results in an undesirably high stray or leakage inductance (typically near 200 micro-henries (μH)) and inefficiencies which inhibit or limit higher frequency audio output.
Also, when the prior art voice coil or solenoid excitation is utilized, the resultant forces exist near or at the periphery or edge of the diaphragm and not uniformly over the diaphragm. Since the diaphragm is typically of a thin, lightweight, high strength and stiffness, and somewhat flexible (preferably dielectric) material such as mylar, it is susceptible to dynamic deformations. (other materials include but are not limited to silk, aluminum, or titanium) As acoustic drive frequencies increase, the diaphragm itself may begin to flex or resonate at its natural frequency or harmonics thereof. That is, since the surface of the diaphragm is not driven directly, i.e. only the edges are driven, the surface of the diaphragm may deform or develop mode resonances to match the edge feed boundary conditions. Obviously, the diaphragm resonance frequencies and harmonics thereof are dependent upon a plurality of factors including but not limited to Young's modulus, density, thickness, etc. of the diaphragm material. Attempts at utilizing an acoustically lossy diaphragm material to dampen said resonances are marginally effective and have reduced acoustic output efficiency and power.
A peripheral diaphragm or dome coil drive also limits the ability of the loudspeaker to acoustically reproduce the drive signal. That is, since the voice coil substantially moves pursuant to the force imputed by the drive current, failure of the diaphragm or dome to fully track the voice coil movement results in a phase distortion or breakup in the acoustic output. Also, the voice coil itself adds undesirable mass which in conjunction with a spring like attachment of the diaphragm with the speaker housing or mount creates a lower frequency resonance.
The aforesaid effects are readily observed when the impedance (typically ordinate axis) of the loudspeaker is plotted versus frequency (typically abscissa axis) within the mechanical and magnetic hysteresis limits of the loudspeaker. That is the impedance seen across the terminals of the voice coil. At lower frequencies, the voice coil resistance is dominant with a free-space resonance (i.e. the aforesaid mass and spring effect) found as the drive frequency increases. Said resonance is primarily due to the diaphragm and solenoid mass interaction with the diaphragm elastic spring equivalence attachment and is usually below one or two kilohertz for an approximately 25 millimeter diameter domed diaphragm. Midrange frequencies generally exhibit a somewhat flat impedance versus frequency characteristic. That is, the reactive or jωL (j is the 90 degree imaginary phase lag mapping operator, ω is the radian frequency, and L is the stray coil inductance) contribution is relatively small compared to the resistive (typically 3.5-8 ohms (Ω)) portion of the solenoid or voice coil. At higher frequencies, the prior art exhibits a primarily inductive reactive response with the complex impedance magnitude following somewhat linearly with frequency except for the aforesaid diaphragm resonance.
When designing loudspeakers for primarily higher frequency audio reproduction (i.e. tweeters) the lower and midrange effects are of less concern since the drive amplifier circuitry, typically a passive or active crossover network, is designed to drive the loudspeaker at higher frequencies. Although undesirable, even the increased and primarily inductive reactive impedance increase verse frequency may be compensated for with a properly designed differentiating amplifier feeding the loudspeaker. That is, the output impedance of the amplifier feed is substantially matched as the complex conjugate of the loudspeaker input impedance. Unfortunately the prior art diaphragm resonance is so unpredictably and non-linear that compensation is difficult on an individual component basis and highly impractical if a repeatable manufacturing process is desired.
Although the prior art diaphragm resonance is often at the limits of the audible range, an audible effect or product may be heard under the proper excitation circumstances. That is, sub-harmonics of the diaphragm resonance may excite the diaphragm resonance which may then combine with other drive frequencies within the non-linear diaphragm material and surrounding mediums to produce audible difference frequencies as inter-modulation distortion. This effect is further exacerbated due to the non-linear complex impedance of prior art voice coils. That is, due to a per cycle movement through regions of varying magnetic flux density, the voice coil inductive reactance changes during portions of each cycle which further introduces inter-modulation products and stimulates a diaphragm resonance. For high fidelity reproduction, this effect is unacceptable. It is also an unacceptable requirement for the speaker drive amplifier to overly compensate for leakage inductance or other anomalies of the speaker.
The present art utilizes a thin and substantially flat diaphragm of a preferably kapton or polyimide material with a planar coil electrical conductor which is etched or deposited (preferably of an aluminium material) thereon and suspended, over a high flux density magnetic field. During operation, the entire planar coil is immersed within the magnetic bias field which minimizes leakage inductance (typically 20 μH). That is, the planar coil current is substantially transferred into diaphragm movement which is seen primarily as a resistive load and not reactive. Also, utilization of a planar voice coil minimizes the prior art solenoid mass and thereby minimizes any lower frequency free-space resonances.
The magnetic field of the present art is further amplified with a neodymium magnetic (i.e. Nd2Fe14B or equivalent with an approximate 1.38 Tesla remnant magnetic flux density or greater) disk placed nearest to the diaphragm within the diaphragm magnetic circuit bias. A uniquely layered strontium ferrite (SrFe12O19) magnet structure positioned below the topmost plane of the neodymium magnet provides an opposite magnetic bias to the neodymium, serves to bend the magnetic flux emanating from the neodymium magnet more orthogonal to diaphragm movement and current flow, and serves to complete the magnetic bias circuit. The present art further acoustically loads the center and periphery of the diaphragm with a high density urethane, felt, polymer, or rubber type material which minimizes diaphragm self resonance or harmonics thereof and the induced diaphragm stretching effect due to any magnetic flux emanation in the direction of diaphragm movement. The present art uniquely drives substantially all of the acoustically active portion of the diaphragm instead of simply the periphery.
The present art is distinguished from prior art ribbon or coaxial ribbon tweeters via utilization of a unique magnetic circuit along with the aforesaid components and an acoustic horn waveguide which maximally matches the acoustic impedance of free space with the diaphragm acoustic impedance and provides a uniform phase field off axis of the speaker central axis. The present art further minimizes the parasitic inductance by a factor of ten (typically 20 μH) relative to the prior art. Unlike the prior art ribbon type tweeters, the present art further presents a direct current (dc) resistive load of approximately 3.5Ω-4Ω which is desirable for conventional amplifier drives.
The fixed waveguide has a central phase plug which sandwiches the central portion of the diaphragm with the neodymium magnet with a layer of urethane, flexible rubber, polymer, or felt like material between each interface as appropriate. The waveguide is constructed of a lossless material and assures that acoustic energy emanating from any circumferential diaphragm position does not travel out of phase to any three dimensional point within the far acoustic field, thereby inhibiting phase distortion. The waveguide sandwich further minimizes the possibility of diaphragm resonance within the audible spectrum. Since the center of the diaphragm is held, any resonance modes must be of a higher order and thereby of a higher frequency for an equivalent diaphragm diameter. As the diaphragm resonance is pushed substantially beyond human perception, even sub-harmonic excitation and intermodulation products produced therefrom are imperceptible.
Accordingly, it is an object of the present invention to provide a planar diaphragm acoustic loudspeaker which minimizes leakage or parasitic inductance, maximizes efficiency, and substantially eliminates acoustically perceptible diaphragm self resonance.
Another object of the present invention is to provide a planar diaphragm acoustic loudspeaker having a uniquely positioned waveguide structure which provides a substantially uniform phase field and minimizes phase distortion off axis.
A further object of the present invention is to provide a planar diaphragm acoustic loudspeaker having a unique magnetic bias structure which maximizes magnetic flux density at a planar coil while positioning said flux maximally orthogonal to diaphragm movement and current flow.
A still further object of the present invention is to provide a planar diaphragm acoustic loudspeaker which substantially flattens the input impedance versus frequency relative to the prior art.
A still further object of the present invention is to provide a planar diaphragm acoustic loudspeaker which minimizes free-space or self resonance.